The remarkable increases in energy costs over the last decade have induced a great deal of developmental effort in conserving energy and more effectively using the energy already available. Almost every device which either generates or utilizes heat energy utilizes some device for transferring heat from where it is generated to where it is to be used. These devices, called heat exchangers, come in a great variety and can involve transferring heat from gases to other gases, gases to liquids, liquids to gases, liquids to liquids, and solids to or from either liquids or gases. The physical characteristics vary widely to adapt to the media (solid, liquid or gas) involved.
Heat exchangers have been known and manufactured for over a century and certain common factors include cooperation with two bodies of different temperatures which are disposed to facilitate the flow of heat from the hotter to the cooler body. In those situations involving two fluids, there is normally a solid barrier between them to prevent intermixing of the two fluids, since if mixing were allowable, a heat exchanger would be unnecessary.
Certain common principles apply to all heat exchangers involving two different fluids. It has long been recognized that, since fluids are good insulators, the transfer of heat requires movement.
Among the factors that affect the efficiency of such heat exchangers is the nature of liquid flow through the unit. In particular, the existence of stagnant films and laminar flow patterns inhibit heat transfer, the effect being especially pronounced in the case of viscous fluids flowing relatively slowly through tubes. It will be appreciated that such conditions will typically exist in the case of shell-and-tube type heat exchangers or double pipe heat exchangers in which one fluid is viscous. In such circumstances, the fluid being heated will, of course, withdraw thermal energy from the hot fluid, thereby cooling the latter and making it more viscous, thus increasing the likelihood that laminar flow patterns and stagnation will occur. This effect is most pronounced at the wall of the barrier between the fluids.
As is well known, devices have been developed for the purpose of inducing turbulent flow, and thereby mixing the fluid as it moves through the heat exchanger. For example, so-called "turbulators" may be fixedly mounted within tubes to produce directional changes, and hence turbulence, in the flowing liquid. Devices for wiping or scraping the walls of the tubes have also been proposed in the interest of disrupting stagnant layers, and are disclosed, for example, in U.S. Pat. Nos. 3,407,871, to Penney, and 4,174,750, to Nichols. However, such devices as are presently known tend to be complex and cumbersome, relatively difficult to install, and limited in their flexibility of application; they may also require an inordinate amount of maintenance. Many even require an outside source of mechanical energy. To an extent, the complexity of such prior devices may be attributed to efforts to minimize friction in the mounting means so as to facilitate rotation, and thereby render them most effective and efficient in terms of power consumption. This, in turn, increases the cost and difficulty of producing and installing such devices and, to that extent, offsets the advantages afforded thereby.
The configurations also limit the application to a specific type of heat exchanger--such as a shell-and-tube heat exchanger or a double pipe heat exchanger, but not both. They further limit the application to new construction, since a retrofit modification of an existing heat exchanger would require custom machine hardware of high cost and costly design.
Accordingly, it is a primary object of the present invention to provide a novel method of creating a heat exchanger having means for preventing stagnation of the fluid against the walls of the tubes through and around which it flows, thereby maximizing the efficiency of thermal energy transfer regardless of the type of heat exchanger, or whether it is an existing or yet-to-be-built heat exchanger.
It is also an object of the invention to provide such a heat exchanger wherein the foregoing advantages are achieved in a manner that is simple, inexpensive and convenient, and in which the need for maintenance is minimized.
Yet another object of the invention is to provide a heat exchanger of the foregoing nature, wherein the wiping effect is independent of the direction of flow of the liquid through the tubes, thereby minimizing difficulty of installation, permitting backflushing of the system in which it is employed, and facilitating the incorporation of such features into single, double and multiple pass units, as well as double pipe heat exchangers.
Yet another object of this invention is to simplify any bearing or anti-friction mechanisms so that a retrofit device can be fashioned for existing heat exchangers, be they single pass, double pass, multipass, or double pipe heat exchangers. To secure maximum effectiveness, special construction is to be avoided and add-on capabilities must be available for existing operating units.